Erosion Rates and Sediment Sources in Madagascar Inferred from 10Be Analysis of Lavaka, Slope, and River Sediment

The central highlands of Madagascar are characterized by rolling hills thickly mantled with saprolite and cut in many areas by dramatic gullies known as lavakas. This landscape generates sediment to rivers via diffusive downslope movement of colluvium and event-driven advection of material from active lavakas; these two sediment sources have very different Be-10 signatures. Analyzed lavaka sediment has very little Be-10 (0.8-10 x 10(5) atoms Be-10 g(-1)), consistent with deep excavation liberating previously shielded saprolite with little exposure to cosmic rays. Colluvium, in contrast, has greater Be-10 concentrations (6-21 x 10(5) atoms Be-10 g(-1)), reflecting long residence times in the near-surface environment. Comparison of Be-10 abundance in hillslope, lavaka, and river sediment samples indicates that lavakas dominate the mass input to rivers (84% by volume) in spite of the fact that they occupy a small fraction of the land surface area. River terrace sediments that are at least a millennium old have Be-10 concentrations indistinguishable from those of modern lavaka-dominated river sands, from which we infer that lavakas were widespread on the landscape at or before the time that humans colonized the central highlands. Erosion rates derived from cosmogenic Be-10 in river sediment average approximately 12 m m. yr.(-1), or about 32 t km(-2) yr(-1), which is three orders of magnitude lower than commonly reported erosion rates for Madagascar.